1. Field of the Invention
The subject invention relates to positioning a laser on a part, and utilizing the laser to mark, or otherwise alter the part, to produce, for example, a graphic in or on the part at a desired position.
2. Description of the Related Art
Methods are known for accurately guiding a laser used to mark, etch, engrave, machine, cut, weld, etc. a workpiece in producing a part, the workpiece perhaps being the resulting part itself prior to or during the process of marking. Herein, the term “mark” and its variants are used to refer not only to marking, but to other alterations of the workpiece performed by a laser, such as but not limited to etching, engraving, machining, cutting or welding. In certain procedures, for example, marking entails producing a graphics on the workpiece, using a process of capturing an image of the workpiece, comparing its position and orientation to a preexisting computer-based model of the part and the desired graphics, relatively oriented to their properly aligned positions, and marking the graphics on the workpiece using a laser to match the graphics positioned on the preexisting computer-based model of the part. The parts in the examples to be discussed herein are automobile side view mirror toggle switch cover plates. The toggle switch cover plate may be molded in white plastic, for example, and painted black. The laser is used to remove the black paint (i.e., mark the workpiece) to mark white arrows in spherical depressions or dimples formed in the toggle switch cover plate. The plate may be molded in a translucent white plastic, the arrows of the part thus being visible at night when backlit by a light source beneath or behind the toggle switch plate.
One such known method used in accomplishing such workpiece marking, which may be undertaken with an apparatus or system 20 as shown in FIG. 1, is intelligent mark positioning (IMP). The apparatus or system 20 of FIG. 1 includes a laser 22, an imaging device such as a camera 24, for instance a CCD (charge-coupled device) or a CMOS (complimentary metal-oxide semiconductor) camera and appropriate lighting, a scan head 26 and a computer 28 having an image display screen 30. The scan head may be a conventional galvanometer or a plurality of galvanometers having a mirror or a plurality of mirrors controlled by the computer. The scan head may also include a lens 34. Such devices are well-know to those or ordinary skill in the art, and are not described in detail herein other than to briefly explain that, in a well-known way, the camera “looks” through the scan head and reads or takes an image of a workpiece disposed beneath the scan head, and the image is directed to the computer, which extracts a geometric vision model of the workpiece to be marked (the “workpiece vision model”) and compares it to a preexisting, stored vision model of the part (the “part vision model”) previously taught to the computer. The part vision model may be of a type having a fixed orientation and position which is continually referred to in registering each new workpiece to be marked (as CAD based registration would employ) or, alternatively, the part vision model may instead be the most recent previous workpiece vision model. Herein, a preexisting CAD-based model, as well as a computer-based model generated/extracted from an image of the part or workpiece, may be referred to as a “vision model” and it is to be understood that that term should be understood to mean either an image-derived vision model, or a CAD-based model.
The image of the workpiece, and the geometric workpiece vision model extracted therefrom include a centrally-located feature FC in the workpiece/part which, in conjunction with another camera-recognized feature such as, for example, a corner 40 or a circle circumscribed by one of the dimples 42 in the toggle switch cover plate, serve orientation and positioning purposes in registering the single vision model of the workpiece to the single vision model of the part, through a process referred to single model registration (SMR). Centrally-located feature FC, which may be a small projection or recess in the part surface, and the other feature recognized in imaging, model generation and registration, are fiducial features common to the part and workpieces to be marked. In the example toggle switch cover plate 60, any one of the corners 40 or the circles circumscribed by dimples 42 may serve with FC as a fiducial feature, owing to the part's symmetry. In a nonsymmetrical part, a feature having a unique orientation relative to FC would preferably be used.
In marking the workpiece, the computer will reorient the galvanometers from a state corresponding to the orientation and position associated with the preexisting part vision model, based on the prior workpiece image, to a state corresponding to the orientation and position associated with the new workpiece vision model, keying on central feature FC and the other fiducial feature, and then load the graphics and align them with the workpiece vision model, again keying on the fiducial features. The galvanometers then guide the laser beam 50 to the unfixtured workpiece 60 located on the work surface (not shown), for marking the graphics on the workpiece at its present position and orientation, which is variable relative to that represented by the preexisting part vision model.
Practicing IMP in accordance with the prior art entails: (A) a teaching phase, and (B) a registration and mark phase. The teaching phase (A) includes the following steps:
(A-1) Capture an image of a workpiece including its fiducial features (see FIG. 2);
(A-2) Generate a part vision model of the captured workpiece image, including its fiducial features. The generation of the part vision model is done automatically, with software that extracts a geometric model of the part from the captured image (see FIG. 3); and
(A-3) Store the part vision model. Alternatively, the user could store a CAD-based part model, eliminating the above steps of capturing an image and generating/extracting a vision model therefrom.
Registration and mark phases (B) include the following steps, with reference to FIG. 4:
(B-1) Capture a new image of the workpiece located on the work surface, including its fiducial features;
(B-2) Generate a new workpiece vision model of the workpiece image, including its fiducial features. Again, the generation of this vision model is done automatically, with software that extracts a geometric model from the new image;
(B-3) Register the new workpiece vision model to the stored part vision model, keying on their respective fiducial features, and generate/extract a transformation matrix which includes position and orientation. Steps B-1 through B-3 relate to the registration phase.
Continuing on to the IMP mark phase, the following steps are carried out, still with reference to FIG. 4:
(B-4) Pass the transformation matrix to the mark engine;
(B-5) Load the graphics; and
(B-6) Mark the workpiece with the graphics using the laser.
A problem with the prior IMP's SMR concept is that it is limited to workpiece features that are visible and detected in the camera field of view, and cannot be applied to featureless parts/workpieces or parts/workpieces where the feature is too large to be detected because it is not visible in the camera view. Although using a vision model far from the mark location is possible, it generates large inaccuracies. Further, SMR is not possible without any feature in the center of the part/workpiece, which is why a fiducial feature such as FC in the above-described example is necessary. Thus, the SMR concept is limited to laser marking methods where the alignment is based on a feature within the camera's field of view.
It is desirable to mark a part or a feature thereof, for example with a graphic, using a laser, the location of the feature or the mark location being aligned with the center of the part, which itself may be featureless, and overcome inadequacies of the prior IMP process and other prior processes employing SMR concepts, including, for example, their shortcomings related to marking features outside of the camera view.